1. Field of the Invention
The invention relates to articles useful for electromagnetic shielding and particularly to structural concrete articles having magnetized reinforcing fibers disposed within the articles wherein a substantial portion of the fibers contact nearby fibers.
2. Description of the Prior Art
Large scale electromagnetic shielding applications require economical shielding elements preferably formed from standard materials such as are used in facility construction. Since facilities are commonly formed of concrete, it is desirable that the concrete itself be made sufficiently electrically conductive to provide required electromagnetic shielding performance. The prior art includes prior attempts to render concrete electrically conductive using steel fibers. For example, Miller et al, in U.S. Pat. No. 4,062,913, orients steel fibers in concrete by means of an electrical field. Similarly, Takazuka, in U.S. Pat. No. 4,159,911, subjects steel fibers to a magnetic field during mixing with concrete with no magnetization of the fibers being developed. Steel fibers have further been mixed in concrete by Dearlove et al, as described in U.S. Pat. No. 4,023,706, and by Akazawa et al, in U.S. Pat. No. 4,121,943. Tani et al, in U.S. Pat. No. 4,804,585, describe the use of steel fibers for simple reinforcement of concrete. Tani et al, as well as Bache in U.S. Pat. No. 4,979,992, provide one of many examples wherein steel fibers are used for reinforcement of concrete but without the capability of providing electromagnetic shielding in the resulting concrete structure. In U.S. Pat. No. 4,615,280, Shoop et al describe a concrete housing for electronic components formed of steel fiber reinforced concrete. As with Tani et al and Bache, the concrete structure is intended to be strengthened by the steel fibers but do not provide an electromagnetic shielding capability of any substantial utility. Accordingly, prior use of steel fibers in concrete has not resulted in concrete articles having substantial electromagnetic shielding effectiveness.
The present invention relates to the reinforcement of concrete articles through the use of metallic fibers and particularly magnetized steel fibers with the resulting concrete articles exhibiting very substantial electromagnetic shielding capability. According to one teaching of the invention, fiber to fiber contact is intended to be continuous at least through substantial portions of the volume of the concrete, this fiber to fiber contact being preferably effected by vibration of the fiber/concrete mix during setting in a form. The present invention thus provides methodology, apparatus and articles of manufacture which provide substantial electromagnetic shielding capability to concrete structures.